In the field of a superconducting cable provided with a superconducting conductor composed of, for example, Bi-based high-Tc superconducting tapes or wires, researchers and engineers have been developing not only a single-phase cable, which is provided with one cable core, but also a multiphase cable, in which a plurality of cable cores are held together to form a multicore-bundled-in-one type cable. FIGS. 2(A) and 2(B) show a three-phase superconducting cable, in which three cores are held together. A superconducting cable 100 has a structure in which three cable cores 102 are twisted together to be housed in a heat-insulated pipe 101. The meaning of the term “wire” used in this Description and in the accompanying Claims is not limited to a wire having a circular cross section. The meaning includes a wire having a rectangular cross section.
The heat-insulated pipe 101 forms a dual pipe composed of an outer pipe 101a and an inner pipe 101b. Although not shown in FIG. 2(A), a heat-insulating material is placed between the two pipes, and the space between the two pipes is evacuated into vacuum. An anticorrosion layer 104 is formed on the outer pipe 101a. Each of the cable cores 102 is provided with a former 200, a superconducting conductor 201, an electrically insulating layer 202, a shielding layer 203, and a protective layer 204 in this order from the center. The superconducting conductor 201 is structured by helically winding superconducting wires in multiple layers on the former 200. The electrically insulating layer 202 is structured by wrapping tapes of semisynthetic insulating paper. The shielding layer 203 is structured by helically winding on the electrically insulating layer 202 superconducting wires similar to those used as the superconducting conductor 201. In the shielding layer 203, an electric current having nearly the same magnitude as that of the current flowing in the superconducting conductor 201 is induced in a steady state and flows in the direction opposite to that of the current in the conductor 201. The magnetic field produced by the induced current cancels out the magnetic field produced by the current flowing in the superconducting conductor 201. As a result, the magnetic field leaking to the outside of the cable core 102 can be decreased to nearly zero. Usually, interstices 103 produced by the inner pipe 101b and the individual cable cores 102 are used as the coolant channel.
When such a multiphase superconducting cable is used to construct a long-distance transmission line, it is necessary to use a joint to joint cable cores drawn out of neighboring cables at some midpoint in the line. An example of such a joint is stated in the Patent Document 1. The structure of this joint is shown in FIG. 3. FIG. 3 shows that ends of the neighboring superconducting conductors 201 housed in a joint box 500 are jointed with each other with a jointing ferrule 510 made of normally conducting material such as copper. A stress relief cone 520 is placed over the end portions of the superconducting conductors 201 and the jointing ferrule 510. The stress relief cone 520 is supported with supporting rods 530 made of fiber-reinforced plastic (FRP). The cable core 102 in the joint box is supported with a supporter 540 made of FRP. (See the Patent Document 1.)
Patent Document 1: published Japanese patent application Tokukai 2000-340274 (FIG. 1)